In a fire alarm system, the function thereof needs to be maintained for several hours even under conditions of a power failure, and emergency bells need to be sounded for a fixed period of time in case of the outbreak of a fire during the power failure. Therefore, a receiver has a built-in emergency power source (storage battery).
In a polling type fire alarm system, terminal devices to be polled by the receiver, such as fire detectors, fire sensors and repeaters have built-in CPU's, the consumption currents of which are not negligibly small. Accordingly, when the CPU's are always held in an operating status, the emergency power source of the receiver must have a large capacity.
It can therefore be considered to curtail wasteful power consumption, in cases where a signal transferred from the receiver or any of the other terminal devices is flowing on a signal line, and in cases where, e.g., the operations of reading and sending fire information from the fire detecting portions (detectors or sensors), fire detectors etc. (repeaters), or the operations of delivering control signals to devices to-be-controlled such as local bells and smoke control equipment, by switching the CPU's built in the terminal devices such as fire detectors from a wait status (under which the arithmetic portion of the CPU's are in standby status) into a run status (under which the arithmetic portion of the CPU's are in an operating status).
Even with this measure, however, the CPU's built in the terminal devices are in a run status while the signal is flowing on the signal line, and in the whole system, the large number of built-in CPU's operate simultaneously during the transmission time of the signal. This leads to the problem that the consumption currents during this time are great.